Radiation-reactive phenylbutadiene multiblock copolymers

ABSTRACT

A method for preparing a substrate coated or laminated with a multiblock copolymer comprising endblocks of phenylbutadiene and an elastomeric midblock of a conjugated diene such as isoprene or butadiene is disclosed. The copolymer is crosslinkable by EB radiation such that the crosslinks are confined primarily to the endblock domains in the polymer with minimal crosslinking occurring in the rubbery matrix. Also disclosed are blends of the copolymers with tackifier resins compatible with the midblock regions. The blends provide a curable PSA having enhanced cohesive strength while maintaining adhesive properties.

This is a division of application Ser. No. 589,139, filed Sep. 27, 1990,now U.S. Pat. No. 5,066,728.

FIELD OF THE INVENTION

The present invention relates to radiation-reactive multiblockcopolymers having an elastomeric midblock and phenylbutadiene endblocks,and more particularly to such copolymers wherein radiation crosslinkingmay be selectively induced in the phenylbutadiene endblock phase.

BACKGROUND OF THE INVENTION

Multiblock copolymers having thermoplastic endblocks and an elastomericmidblock are well known from the prior art. Such block copolymersinclude, for example, polystyrene/polyisoprene/polystyrene (SIS) andpolystryene/polybutadiene/polystyrene (SBS). Because these polymersexhibit both thermoplastic properties owing to the thermoplasticendblocks, as well as elastomeric properties due to the elastomericinternal block, these polymers have found a wide variety of commercialapplications. One such commercial application employs the multiblockpolymer in a blend with a tackifying resin for pressure sensitiveadhesive (PSA) compositions. The composition is typically applied to afilm, label, tape or other substrate to adhere the film, label, tape orsubstrate to yet another substrate in a laminated structure.

It is known that radiation-induced crosslinking of PSAs can be utilizedto enhance cohesive strength, e.g. temperature performance. For example,the use of multifunctional acrylate monomer crosslinking agents isdisclosed in U.S. Pat. No. 4,133,731 to St. Clair et al. However, thevolatility and toxicity of such monomers present hazards and processingdifficulties in the manufacturing process.

It is also known in the art to functionalize polymers to incorporateinherent radiation reactivity. However, such functionalization methodsare typically multistep processes involving complex preparationtechniques.

A major challenge in the design of radiation cross-linkable multiblockcopolymers suitable for PSA applications is balancing cohesive andadhesive strength properties of the PSA after irradiation. Ordinarily,gains achieved in enhancing shear properties (cohesive strength) byintroducing radiation-activated crosslinks is offset by sacrifices inpeel and tack properties (adhesive strength). In St. Clair '731, forexample, acrylate crosslinking agents are stated to partition betweennon-elastomeric endblocks and elastomeric midblocks in SIS and SBStriblock copolymers. Solubility factors favor preferential crosslinkingby the acrylate monomers in the polystyrene blocks, however, reactivityconsiderations still favor crosslinking in the rubbery midblocks.

Further efforts involve the use of multifunctional acrylate monomers tocrosslink triblock copolymers such as styrene/ethylene-1-butene/styrene(SEBS) block copolymers and block copolymers with a polydiene midblock.The elastomeric blocks are hydrogenated to reduce the occurrence ofcrosslinking in the rubbery matrix, as disclosed in U.S. Pat. Nos.4,151,051 and 4,152,231 to St. Clair et al.

A further technique disclosed in U.S. Pat. No. 4,566,464 to St. Clair etal. involves the incorporation of unsaturation into the polystyreneblocks and the use of multifunctional acrylate monomer coupling agents.This is said to enable efficient crosslinking within the polystyrenedomains, but the rubbery midblocks still contain relatively high levelsof residual unsaturation so that crosslinking in this region has adverseeffects on tack properties.

The radiation response of poly(2-phenylbutadiene) is disclosed inYamaoka et al., "Primary Processes in the Radiation-Induced Crosslinkingof Poly(2-Phenylbutadiene)", Polymer Journal, Vol. 19, No. 5, pp.667-672 (1987). The polymer is stated to contain 88 weight percent of1,4-units, 6 weight percent of 1,2-units and 6 percent of 3,4-units. Thecrosslinking is said to occur under gamma-radiation with a G_(crosslink)(the number of crosslinks per 100 eV radiation energy absorbed) of 7.2.This high value was theorized to be due to benzyl cation and allylradical intermediates, both of which lead to crosslinking.

SUMMARY OF THE INVENTION

It has been discovered that a block copolymer comprising endblocks ofphenylbutadiene and an elastomeric midblock of a conjugated diene may beradiation crosslinked to give a block polymer having crosslinks confinedprimarily to the endblock domains. The use of phenylbutadiene as theendblock monomer solves a key problem present in the prior art, namely,preferential crosslinking of the endblocks without substantiallycrosslinking the elastomeric midblocks. Solubility parameter differencesbetween uncured polyphenylbutadiene and polyisoprene or polybutadiene,for example, give discrete two-phase morphology. Subsequent radiationexposure preferentially crosslinks the polyphenylbutadiene phase becauseof substantially greater G_(crosslink) of the polyphenylbutadienecompared to G_(crosslink) of the elastomeric midblock. The presentmultiblock copolymer may be prepared in a single polymerization step andcrosslinked without the use of crosslinking additives such asmultifunctional acrylate and methacrylate monomers.

Broadly, the present invention provides a radiation-reactive multiblockcopolymer, comprising endblocks comprising polymerized monomer of theformula: ##STR1## wherein at least one of R₁ and R₂ is phenyl orhydrocarbyl-substituted phenyl, and one of R₁ and R₂ can be hydrogen;and an elastomeric midblock of a conjugated diene. Preferably, theendblock has a weight average molecular weight (M_(w)) of from about10,000 to about 20,000, and the midblock has a M_(w) of from about100,000 to about 150,000. The endblocks preferably comprisepoly(2-phenylbutadiene) and the midblocks preferably comprisepolyisoprene or polybutadiene.

In another embodiment of the present invention, a cured multiblockcopolymer comprises the product obtained by exposing the multiblockcopolymer to a quantity of radiation effective to form crosslinks in theendblocks. The copolymer preferably comprises endblocks of crosslinkedpoly(2-phenylbutadiene) and midblocks of polyisoprene or polybutadieneessentially free of crosslinking.

In a further embodiment, the present invention comprises an adhesivecomposition comprising the multiblock copolymer blended with atackifying resin which is compatible with the midblock. The copolymerpreferably comprises endblocks of poly(2-phenylbutadiene) and a midblockof polyisoprene or polybutadiene.

In yet another embodiment, the present invention includes a method ofcrosslinking the adhesive composition comprising the steps of coating atleast a portion of a substrate with the adhesive composition andirradiating the coating with radiation sufficient to crosslink theendblock. The adhesive composition preferably comprises a multiblockcopolymer having poly(2-phenylbutadiene) endblocks and polybutadiene orpolyisoprene midblocks and blended with a tackifying resin which iscompatible with the midblock.

DETAILED DESCRIPTION OF THE INVENTION

The radiation-reactive copolymer of the present invention is amultiblock copolymer generally of the formula A-B-A wherein A representsa non-elastomeric polymer endblock segment and B represents anelastomeric polymer midblock segment. The multiblock polymer mayalternatively have a star block configuration and/or include severalinitiation, transition and termination blocks conventional in the artfor multiblock copolymers. The molecular weight of the block copolymergenerally ranges from about 60,000 to about 350,000, preferably fromabout 120,000 to about 250,000.

Endblocks comprise a block homopolymer or copolymer prepared frommonomers having the general formula: ##STR2## wherein at least one of R₁and R₂ is phenyl or a hydrocarbyl-substituted phenyl, preferably phenylor lower hydrocarbyl-substituted phenyl and one of R₁ and R₂ can behydrogen. Examples include 2-phenylbutadiene, 1-phenylbutadiene,2-(2,4-dimethylphenyl)butadiene, 2-(4-methylphenyl)butadiene,2-(4-ethylphenyl)butadiene, and the like. A preferred monomer is2-phenylbutadiene and for the purposes of illustration,2-phenylbutadiene is referred to hereinafter with the understanding thatother phenylbutadienes may be similarly employed. The endblocks may alsoinclude a relatively minor proportion of non-elastomeric monomers suchas, for example, styrene, para-methylstyrene, t-butylstyrene, vinylnaphthalene and the like. Glass transition temperature (T_(g)) of theendblocks is generally greater than 0° C. usually in the range of about0°-50° C. The molecular weight of each of the endblocks generally rangesfrom about 5000 to about 50,000, preferably from about 10,000 to about20,000.

The elastomer midblock may be a homopolymer of a conjugated diene havingup to about 12 carbon atoms, preferably from about 4 to about 8 carbonatoms, or a block copolymer of 2 or more such dienes. Examples of suchmidblock monomers include butadiene, isoprene, dimethylbutadiene,piperylene, butyloctadiene, and the like. M_(w) of the elastomeric blockgenerally ranges from about 50,000 to about 250,000, preferably fromabout 100,000 to about 150,000. T_(g) of the midblocks is generally lessthan 0° C.

The block copolymers of the present invention are inherentlyradiation-reactive requiring no crosslinking additives. Solubilitydifferences between poly(2-phenylbutadiene) endblocks and the midblockelastomer enables the formation of discrete two-phase morphologynecessary for PSA formulations having enhanced performance.Surprisingly, the G_(crosslink) value for poly(2-phenylbutadiene) ismuch greater than the G_(crosslink) for isoprene and butadiene basedrubbers, and therefore, crosslinking in the block copolymer occurspreferentially in the endblock phase. Such block copolymers can produceexcellent PSAs which, upon radiation crosslinking, have enhancedcohesive strength and temperature performance without sacrificingadhesive properties.

In the practice of the present invention, a suitable tackifying resincompatible with the elastomeric midblock may be blended with themultiblock copolymer to obtain an adhesive formulation. Tackifyingresins provide enhanced adhesive properties for use in adhesivecompositions such as hot melts and pressure sensitive adhesives (PSAs).Tackifying resins are used to modify viscoelastic properties of theblock copolymers, and to obtain the desired overall T_(g) for theadhesive composition, depending on the type of application.

As suitable tackifiers, there may be mentioned terpene resins, aliphaticresins, and the like. Such tackifying resins may be used alone, or acombination of two or more tackifying resins may be used in the sameblend. The tackifiers are selected for their ability to tackify theelastomeric block and compatibility therewith. Tackifiers compatiblewith polyisoprene include those derived from the copolymerization ofdiolefins, and especially C₅ diolefins such as piperylene with C₅olefins such as 2-methyl-2-butene. These resins, such as ESCOREZ 1310LCand ESCOREZ 5280 available commercially from Exxon Chemical, have ringand ball softening points between 80° and 115° C. A tackifier compatiblewith polybutadiene includes WINGTACK 95 which is the tradename for adiene-olefin copolymer of piperylene and 2-methyl-2-butene having asoftening point of 95° C. Another useful resin, ZONATAC 105 Lite,available from Arizona Chemicals, is prepared by the cationicpolymerization of limonene and styrene. The tackifying resins aretypically present at from 50 to 200 parts by weight per 100 parts of theblock copolymer, preferably from 75 to 150 parts by weight.

Hydrocarbon extending oils can be employed in this application to modifythe formulation viscosity and to increase the tackiness of the adhesive.The extending oils, referred to as paraffinic/naphthenic oils arefractions of refined petroleum products having less than 30 percent byweight aromatics and viscosities ranging from 100 to 500 SSU at 100° F.Such oils are readily commercially available, such as SHELLFLEX 371, anaphthenic oil manufactured by Shell.

The blend may further contain conventional additives such as, forexample, stabilizers, dyes, pigments, fillers and the like, but theblend should be free of other additives and impurities in amounts whichadversely affect the adhesive properties of the blend, and particularlythe high temperature adhesive properties thereof.

The endblock monomer 2-phenylbutadiene, for example, may be synthesizedfrom a corresponding alcohol obtained by a Prins reaction ofα-methylstyrene or by the Grignard reaction of acetophenone.

Preparation of the multiblock copolymer is by anionic polymerizationmethods well known in the art. Polymerization proceeds by the successiveadditions of monomer to be polymerized in order of incorporation intothe polymer. Further control over the polymer composition molecularweight distribution and architecture may be gained by utilizing alinking agent to link together two different "living" polymer chaincenters comprising, for example, an end and midblock section, i.e.A-B-B-A. In this manner, geometric architecture other than linearpolymers, such as star-shaped polymers, are possible. Linear polymersare joined in this process, for example, by dichlorodialkylsilanecompounds, or any silane compounds having two reactive moieties attachedto the silicon atom. Star-shaped polymers may be prepared by utilizingtri- or tetra-halide substituted silane compounds and the like. In thefirst sequence, endblock monomer is added to a suitable solvent mediumcontaining the initiating reagents. Next midblock monomer is added,followed by other additional endblock monomer or a silane linking agent.

Polymerization conditions are employed which are typical for anionicpolymerization. Initiators may also be selected from initiators commonlyemployed in anionic polymerization methods. Generally, these areorganometallic compounds derived from alkali metals. The preferredalkali metal compounds are organolithium compounds. n-Butyl lithium isespecially preferred. The amount of initiator used is generally based onthe amount of monomer used to form the first sequence and may varywidely depending on the type of initiator and type of monomer.Generally, however, about 0.1 to about 100 millimoles of initiator perhundred grams of monomer is required.

The polymerization process is generally conducted in an organic solvent.Both polar and nonpolar solvents are suitable, such as ethylenedichloride or benzene.

Polymerization temperature depends on initiator, monomer and solventselection with the overall intent to maintain the "living" character ofthe chain center and retard side reactions which will broaden themolecular weight distribution and reduce molecular weight. Generally,the polymerization can be undertaken from about -100° C. to about 100°C. Typically the polymerization is carried out under sufficient pressureto maintain the monomers in a liquid phase. Polymerizationconventionally proceeds under an inert atmosphere.

Generally, the "living" block copolymers are deactivated at the end ofpolymerization by conventional means such as the addition of an alcoholor carboxylic acid. Also, as mentioned previously, deactivation may beaccomplished by the coupling of a "living" chain center with a couplingagent such as a disubstituted halo-silane or similar reactive compound.Thereafter the multiblock copolymer is filtered, recovered and driedunder vacuum. Polymerization may be conducted in any suitable vessel.

The adhesive compositions of the present invention may be prepared byany method which results in intimate admixture of the components.Preferably, a blend is prepared by dissolving the components in asolvent such as toluene and/or mixing solutions of the components,followed by evaporation of the solvent(s). Alternatively, dry mixing,such as, for example, on a mill, may also be employed, but this willgenerally require heating the components and/or the mill to obtain asuitable degree of intimate admixture.

The adhesive compositions can be employed by applying a solution or hotmelt of the composition to a substrate using equipment conventional inthe art. A solution of up to about 40 weight percent solids in a solventsuch as toluene is prepared and the solvent is removed by evaporation.Alternatively, the ingredients may be mixed in a solvent, the solventevaporated to form an aqueous emulsion and the adhesive may be appliedto the substrate as a 50-60 weight percent solids water-based emulsion,the water being removed by subsequent evaporation.

The adhesive may be applied as a hot melt where the formulated blockcopolymer has a relatively low processing viscosity, e.g. less thanseveral hundred thousand centipoise at processing temperatures of up toabout 150° C. The copolymer as a hot melt adhesive may be supplied to asubstrate surface by the use of an extruder to feed a coating die.

Suitable substrates include, for example, plastics, metals, wood, paper,rubber, glass, fabrics such as carpet and canvas and the like. Theabove-described adhesive composition is generally deposited in adhesivecontact with at least one surface of a substrate to form a structure.For example, laminates may be formed by bonding opposed surfaces ofsubstrates with the above-described adhesive blend.

A preferred use of the present invention is the preparation of pressuresensitive adhesive tapes or labels comprising a tape or label substratehaving at least one surface to which the above-described adhesivecomposition is applied wherein the adhesive composition is tacky at roomtemperature. The pressure-sensitive adhesive tape comprises a flexiblebacking sheet and a layer of adhesive composition of the novel PSAcompound coated on one major surface of the backing sheet. The backingsheet may be a plastic film, paper or any other suitable material andthe tape may include various other layers or coatings, such as primers,release coatings and the like, which are used in the manufacture ofpressure-sensitive tapes. According to the "Glossary of Terms Used inthe Pressure Sensitive Tape Industry", a pressure sensitive adhesive isa material which is aggressively and permanently tacky, adheres withoutthe need of much more than finger pressure, exerts a strong holdingforce, and has sufficient cohesiveness and elasticity that it can beremoved from substrates without leaving a residue. A hot melt adhesive,on the other hand, is a 100 percent nonvolatile thermoplastic materialthat is heated to a melt and applied to the substrate as a liquid. Thehot melt bond forms after the liquid cools and solidifies. Some pressuresensitive adhesives, such as those based on block copolymers, areapplied as hot melts, and are referred to as hot melt-pressure sensitiveadhesives.

The adhesive compositions of the present invention can be cured byexposure to high energy electromagnetic radiation such as electron beamradiation or gamma radiation. Sources of such radiation include anatomic pile, a resonant transformer accelerator, a Van de Graff electronaccelerator, a Linac electron accelerator, a betatron, a synchrotron, acyclotron, or the like. Radiation from these sources will produceionizing radiation such as electrons, protons, neutrons, deuterons,gamma rays, X-rays, α-particles and β-particles.

The crosslinking reaction is conveniently effected at room temperature,but it can be conducted at depressed or elevated temperatures ifdesired. It is also within the spirit and scope of the invention toeffect the crosslinking reaction within the confines of an inertatmosphere to prevent air inhibition of the crosslinking reaction and toprevent oxidative degradation of the polymer. The amount and kind ofradiation required depends primarily on the level of curing desired.Generally, the radiation dosage should be effective to crosslink thephenylbutadiene endblocks, but should not be excessive such that themidblocks remain essentially free of crosslinking to the extent thatadhesive properties (e.g. tack and peel) are not substantially adverselyaffected. Suitable doses of EB or gamma radiation are from about 0.2megarad to about 20 megarad, preferably from about 1 megarad to about 10megarad.

The invention is illustrated by way of the following examples.

EXAMPLE 1

Poly(2-phenylbutadiene-isoprene-2-phenylbutadiene) multiblock copolymer(PIP) is prepared by the anionic polymerization of monomers sequentiallyadded so that the molecular weight of the endblocks is in a range fromabout 10,000 to about 20,000 and the midblock molecular weight rangesfrom about 100,000 to about 150,000. Prior to polymerization, thefollowing purification steps are carried out:

1. 2-Phenylbutadiene monomer is prepared by the dehydration of thealcohol obtained by the Prins reaction of α-methylstyrene (or by theGrignard reaction of acetophenone), and then vacuum distilled prior touse.

2. Benzene and isoprene are refluxed with metallic sodium, potassium andbenzophenone to purify, then distilled prior to use.

3. 1,1-Diphenylethylene co-initiator is distilled prior to use.

In a sealed reaction vessel under constant stirring and a nitrogenatmosphere, purified benzene 1,1-diphenylethylene and n-butyllithium aresequentially added by syringe. The solution is dark red in color. Afterfive minutes, an initial quantity of 2-phenylbutadiene is injected intothe reaction vessel and allowed to react overnight at room temperature.A quantity of purified isoprene is added and the polymerization proceedsfor 24 hours. A final addition of 2-phenylbutadiene is made andpolymerization conducted for another 24 hours followed by terminationwith methanol. Following precipitation from excess pentane, filtrationand vacuum drying, a polymer is obtained having poly(2-phenylbutadiene)endblocks of M_(w) 10,000-20,000 and a polyisoprene midblock of M_(w)100,000-150,000.

EXAMPLE 2

In this example, a poly(2-phenylbutadiene-butadiene-2-phenylbutadiene)(PBP) multiblock copolymer is prepared as in Example 1, except thatbutadiene is used in place of the isoprene.

EXAMPLES 3 and 4

Adhesive compositions are prepared using the block copolymers ofExamples 1 and 2. The PIP copolymer (Example 1) is blened with 100 phrof ESCOREZ 1310LC which is compatible with the polyisoprene midblock.The PBP copolymer (Example 2) is blended with 100 phr of WINGTACK 95tackifier which is compatible with the polybutadiene midblock. Theresulting adhesive formulations are dissolved in toluene and knifecoated on MYLAR substrates and the solvent is allowed to evaporate toproduce PSA coatings. Following crosslinking with EB radiation, PSAtests are performed to determine the cohesive (shear adhesion failuretemperature and holding power) and adhesive (rolling ball tack and peel)strength properties. Cohesive properties are found to be enhanced whilemaintaining good adhesive properties.

The foregoing description of the invention is illustrative andexplanatory thereof. Various changes in the materials, apparatus, andparticular parts employed will occur to those skilled in the art. It isintended that all such variations within the scope and spirit of theappended claims be embraced thereby.

What is claimed is:
 1. A method for preparing a substrate coated with acured multiblock copolymer coating, comprising the steps of:(A) coatingat least a portion of a substrate with a multiblock copolymercomprising:(1) endblocks comprising the polymerized monomer of theformula; ##STR3## wherein at least one of the R₁ and R₂ is phenyl orhyrocarbyl-substituted phenyl, and one of R₁ and R₂ can be a hydrogen;and (2) an elastomeric mid block of a conjugated diene; and (B)irradiating said coating with a radiation sufficient to cross link saidend blocks.
 2. The method of claim 1, wherein said irradiation stepcomprises exposing said coating to from about 0.2 to about 20 megaradsof electron beam or gamma radiation.
 3. The method of claim 1, whereinsaid exposure is from about 1 to about 10 megarads.
 4. The method ofclaim 1, wherein said multiblock copolymer in said coating step isblended with a tackifier compatible with said midblock.
 5. The method ofclaim 4, wherein said blend in said coating step comprises saidmultiblock copolymer and said tackifier in a solvent.
 6. The method ofclaim 4, wherein said blend in said coating step comprises themultiblock copolymer and the tackifier in an emulsion.
 7. The method ofclaim 4, wherein said blend in said coating step comprises themultiblock copolymer and the tackifier in a hot melt.
 8. A coatedarticle comprising:a substrate coated with a cured multiblock copolymercomprising:(1) end blocks comprising polymerized monomer of the formula;##STR4## wherein at least one of R₁ and R₂ is phenyl orhydrocarbyl-substituted phenyl and one of R₁ and R₂ can be hydrogen; and(2) an elastomeric mid block of a conjugated diene.
 9. A laminate,comprising:first and second substrates having opposed surfaces inadhesive contact with an interposed layer of a cured multiblockcopolymer comprising:(1) endblocks comprising polymerized monomer of theformula: ##STR5## wherein at least one of R₁ and R₂ is phenyl orhydrocarbyl-substituted phenyl and one of R₁ and R₂ can be hydrogen; and(2) an elastomeric mid block of a conjugated diene.
 10. The laminate ofclaim 9, wherein the interposed layer comprises a tackifier blended withsaid copolymer.